Nitrogen trifluoride (NF3) has been employed for such products as high energy liquid and solid propellants and as a fluorine source for the production of semiconductor devices. Nitrogen fluoride can be prepared by a variety of processes such as disclosed in U.S. Pat. Nos. 4,091,081 and 5,637,285 and U.S. Patent Publication No. 2002/0127167 each of which is incorporated herein by reference. The production of nitrogen trifluoride generally results from a series of reactions as described below in which ammonia and fluorine gas are combined with a liquid ammonium acid fluoride intermediate.
Reaction 1 represents the desired NF3 production reaction. Reaction 2 is the most rapid undesirable side reaction. Reaction 3 represents the fluorine gas that does not react.
                    Reaction        ⁢                                  ⁢        1        ⁢                  :                ⁢                                  ⁢        3        ⁢                  c                      1            ⁢                                                                ⁢                              F            2                    ⁡                      (            g            )                              +                        [                                    x              +              4                                      x              +              1                                ]                ⁢                              NH            3                    ⁡                      (            g            )                              +              α        ⁢                                  ⁢                              c            1                    /          3                ⁢                                  ⁢                  NH          4                ⁢                              F            ⁡                          (              HF              )                                x                ⁢                  (          l          )                      ->                            c          1                ⁢                  NF          3                    +                                    c            1                    ⁡                      [                                          α                3                            +                              3                                  x                  +                  1                                                      ]                          ⁢                  NH          4                ⁢                              F            ⁡                          (              HF              )                                x                ⁢                  (          l          )                                        Reaction        ⁢                                  ⁢        2        ⁢                  :                ⁢                                  ⁢        3        ⁢                  c                      2            ⁢                                                                ⁢                              F            2                    ⁡                      (            g            )                              +                                    c            2                    ⁡                      [                                          8                +                                  2                  ⁢                  x                                                            x                +                1                                      ]                          ⁢                              NH            3                    ⁡                      (            g            )                              +                        c          2                ⁢                  α          ⁢                                          /          3                ⁢                                  ⁢                  NH          4                ⁢                              F            ⁡                          (              HF              )                                x                ⁢                  (          l          )                      ->                            c          2                ⁢                                  ⁢                  N          2                    +                                    c            2                    ⁡                      [                                          α                3                            +                              6                                  x                  +                  1                                                      ]                          ⁢                  NH          4                ⁢                              F            ⁡                          (              HF              )                                x                ⁢                  (          l          )                    Reaction 3: c3 F2(g)→c3 F2(g)This simplified representation assumes that Reaction 1, Reaction 2, Reaction 3 are the only fluorine consuming reactions (c1+c2+c3=1) and neglects slower, but important, fluorine reactions that produce N2F2 and N2F4, OF2, and other impurities. The c1 and c2 values are the estimated fractions of the fluorine feed that react to produce respectively, the desired NF3 product and the undesirable N2 by-product. The c3 fraction represents the fluorine gas that does not react. The NH4F(HF)x melt acidity x value is the molar ratio of hydrogen fluoride to ammonia minus one in the melt. The α value is the ratio of the actual liquid ammonia acid fluoride melt [NH4F(HF)x] feed rate to the minimum stoichiometric NH4F(HF)x feed rate. The temperature of the reaction zone is moderated by using excess NH4F(HF)x. This simplified process representation neglects other factors, e.g. the small fraction of HF in the crude NF3 vapor product. Despite these limitations Reaction 1, Reaction 2, Reaction 3 provide a good framework to describe the state of the art for NF3 production and the problems associated therewith as discussed below.
Efforts have been made to increase the conversion of F2 to NF3. U.S. Pat. No. 4,091,081 discloses the production of NF3 by sparging fluorine and ammonia into liquid ammonium acid fluoride in a column. U.S. Pat. No. 5,637,285 discloses that simultaneously increasing the hydrogen fluoride content of the ammonium acid fluoride melt and increasing the mixing intensity increases the F2-to-NF3 conversion rate. In particular, increasing the NH4F(HF)x melt acidity “x” value decreases the rate of Reaction 2 relative to Reaction 1 which tends to increase the overall NF3 yield. However, higher NH4F(HF)x melt acidity “x” values also somewhat decrease the rate of NF3 production via Reaction 1 for a given interfacial surface area.
Although U.S. Pat. No. 5,637,285 achieves improvements in NF3 yield, some significant problems remain. First, the highly exothermic NF3 reaction is most rapid at the impeller tip, which causes localized overheating, accelerated corrosion at the impeller tip, and mechanical reliability problems. Second, one can not increase the solution depth, to increase the gas residence time, without increasing the mixing shaft length, which also increases mechanical reliability problems.
United States Patent Application Publication No. US/2002/0127167 A1 employs the advantages of higher mixing intensity as disclosed in U.S. Pat. No. 5,637,285 and higher circulation rate to increase the effective α value to decrease localized overheating, increase F2-to-NF3 conversion (c1), and decrease the impeller corrosion. However, this approach does not address the issue concerning the impeller shaft length or the combination of high mixing rates and high fluorine concentration. In addition, the NH4F(HF)x melt recirculation rate is limited by gaseous product flow rate and gas-liquid separation.
It would therefore be a significant advantage in the art to provide a NF3 production system with improved yields without the problems associated with corrosion of the mixing assembly (e.g. impeller) due to the corrosive nature of the fluorine gas reactant and high shear rates.